The present invention relates in general to processing webs of paper, plastics and other materials with calender rolls and, more particularly, to a method and apparatus for controlling one or more calender rolls to control characteristics of such webs. The present invention is initially being applied to the manufacture of webs of paper and, accordingly, will be described herein with reference to this application.
In manufacturing webs of material, such as paper, a variety of characteristics of the web can be controlled by passing the web through a nip formed between two cooperating pressing surfaces, such as counter-rotating pressure rolls or calender rolls. For example, the caliper, density and surface characteristics of a web of paper can be controlled by means of passing the web through calender rolls. To make the web caliper uniform across its width or in the cross-machine direction, the diameters at consecutive longitudinal zones along one or more of the calender rolls are controlled. The rolls are typically constructed of a material having positive thermal expansion such that the rolls expand when heated and contract when cooled. The diameters of calender rolls are then controlled by individually heating and cooling the longitudinal zones along the rolls.
A variety of actuator control arrangements have been used in calender rolls. In one instance, induction heating has been applied for rapidly heating longitudinal zones of a calender roll; however, cooling with this arrangement tends to be slow. More conventionally, conditioned air has been directed against longitudinal zones of a calender roll. Hot or cold air or mixtures of hot and cold air have been blown onto the longitudinal zones of calender rolls to control their diameters.
In U.S. Pat. No. 4,984,622, which issued to Reed, the hot and cold air is blown through a flow passageway which extends circumferentially around a calender roll and is defined by an arcuate scoop which is concentric with the roll. When hot and cold air are thus blown separately onto a calender roll at constant pressure, the air velocities are different due to the differing densities of hot and cold air. The differing air velocities cause air turbulence at the boundaries between the longitudinal zones such that the air flow in each zone affects the zones on both of its sides. This widens the effect of each zone on the calender roll and reduces the magnitude of the effect near the center of each zone. Thus, while Reed is an effective control for calender rolls, ideally, the boundaries between the zones should be crisp with little turbulence to increase the control at each zone and reduce the interference between zones.
While attempts have been made to provide constant volume air flow in calender roll actuators, for example by air mixing, problems remain in existing calender roll actuators. Accordingly, a need remains for an improved actuator and actuator control arrangement for calender rolls. Preferably, this arrangement would substantially equalize the velocities of air which flows circumferentially across calender rolls by raising the cooling air velocity to match the typically higher hot air velocity to improve heat transfer during cooling as well as narrow the effective widths of the longitudinal control zones along calender rolls.